Just to make thing EZ, I would stroke the 340 with a 4 inch (or greater) crank. Equipe the engine with the best and most well done heads as possible. Headers IMO should be at least stepped or straight to a 1-3/4 tube. Cam intake and carb as you need.
Going the B/RB route is great on a lot of levels. I have done it. It would be worth it and best when the LA option is at its limits and only big cubes solve the problem that the LA can’t do.
Don’t get rid of the big blocks. Store and save them. They may yet be useful to you later.
